If the concrete is in good shape (no cracks or flaking) and it has the right dimensions, why not. Concrete gets stronger as it ages and the underlying soil has likely settled in those years. Just pour a new floor over the top after wetting it down well.

I'd start with the cow-shed.. If anything, keep it simple and low cost.You can always make it a guest house later and build a new house.That way, you also get building experience and know your contractor's work before starting the 'real deal'.Cheers,Luc